The present invention relates to an improvement in a bearing for rotatable shafts, in particular in turbochargers, comprising antifriction bearings located between shaft and casing to accommodate radial forces and axial forces acting in one direction on the shaft, and a plain bearing to accommodate axial forces acting on the shaft in the opposite direction, and a lubricating-oil supply system.
In thermal turbomachines, as also in turbochargers, large axial forces can occur which, if antifriction bearings are used in view of friction loss and performance under conditions of insufficient lubricant, are best accommodated by angular, i.e., inclined contact ball bearings or tapered roller bearings. Normally, the axial force always acts in the same direction, but relatively small negative forces can arise under certain operating circumstances, especially when turbochargers are operated intermittently. It two inclined contact ball bearings are arranged in a turbocharger in an "X configuration" with respect to each other, the negative forces can be accommodated by the rear bearing. If very high positive axial forces occur, both inclined contact bearings are mounted in tandem arrangement and act in the same direction. In this case there is scarcely room in most turbochargers for a third bearing arranged the other way round, and if such a bearing were to be fitted, the oil supply and the dissipation of heat would present serious difficulties.
In known turbochargers, four-point bearings are often used instead of inclined contact bearings, as these can accommodate both positive and negative axial forces. However, difficulties are encountered in manufacturing and fitting such four-point bearings in turbochargers with small axial clearances and high operating temperatures, since with this configuration there is a tendency for the bearing to seize under certain circumstances. Furthermore, extremely high accuracy of manufacture is required and an asymmetrical construction is necessary in order to keep the bearing loss as low as with inclined contact bearings and to facilitate the flow of oil through the bearing. The design is therefore considerably more complex and costly.
Various combinations of antifriction bearings and plain bearings have also been employed. These are then arranged so that the load or the speed is distributed between the two types of bearing.